Mid-Gap States and Normal vs Inverted Bonding in Luminescent Cu⁺- and Ag⁺-Doped CdSe Nanocrystals

Abstract

Mid-gap luminescence in copper (Cu)-doped semiconductor nanocrystals (NCs) involves recombination of delocalized conduction-band electrons with copper-localized holes. Silver (Ag)-doped semiconductor NCs show similar mid-gap luminescence at slightly (∼0.3 eV) higher energy, suggesting a similar luminescence mechanism, but this suggestion appears inconsistent with the large difference between Ag and Cu ionization energies (∼1.5 eV), which should make hole trapping by Ag highly unfavorable. Here, Ag-doped CdSe NCs (Ag:CdSe) are studied using time-resolved variable-temperature photoluminescence (PL) spectroscopy, magnetic circularly polarized luminescence (MCPL) spectroscopy, and time-dependent density functional theory (TD-DFT) to address this apparent paradox. In addition to confirming that Ag:CdSe and Cu:CdSe NCs display similar broad PL with large Stokes shifts, we demonstrate that both also show very similar temperature-dependent PL lifetimes and magneto-luminescence. Electronic-structure calculations further predict that both dopants generate similar localized mid-gap states. Despite these strong similarities, we conclude that these materials possess significantly different electronic structures. Specifically, whereas photogenerated holes in Cu:CdSe NCs localize primarily in Cu(3d) orbitals, formally oxidizing Cu to Cu, in Ag:CdSe NCs they localize primarily in 4p orbitals of the four neighboring Se ligands, and Ag is not oxidized. This difference reflects a shift from "normal" to "inverted" bonding going from Cu to Ag. The spectroscopic similarities are explained by the fact that, in both materials, photogenerated holes are localized primarily within covalent [MSe] dopant clusters (M = Ag, Cu). These findings reconcile the similar spectroscopies of Ag- and Cu-doped semiconductor NCs with the vastly different ionization potentials of their Ag and Cu dopants.

The Myth of d8 Copper(III)

“…Dimethyl sulfoxide (DMSO), 2,2′-bipyridine (bpy), anhydrous copper dichloride (CuCl 2 ), copper dibromide (CuBr 2 ), 2-mercaptopyridine, potassium hydroxide, iodoform, tetra-nbutylammonium bromide, ethyl acetate, and hexane were purchased from VWR International, Sigma-Aldrich, and Fisher Scientific and used without further purification. 12, 40 24 and ( EMind L)Cu(N-(C 6 H 4 t Bu)) (17) were prepared according to literature procedures.…”

Explaining the Unusual Photoluminescence of Semiconductor Nanocrystals Doped via Cation Exchange

“…This is expected behavior as this peak is attributed to the presence of the Ag + as a dopant within the CdSe lattice. 25,37 EFM is an exceptionally sensitive technique that can be used to measure the charged state of single NCs to less than 1 elementary charge e. 40,41,42,43,44 EFM is a modification of atomic force microscopy that works by measuring the long-range electrostatic forces between a conductive cantilever and a conductive substrate. In our EFM experiment, a first pass of the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 cantilever measures the topography of the sample.…”

Thioaluminogermanate M(AlS2)(GeS2)4 (M = Na, Ag, Cu): Synthesis, Crystal Structures, Characterization, Ion-Exchange and Solid-State 27Al and 23Na NMR Spectroscopy

“…The wide range absorption behavior of this compound may originate from mid-gap states or result from compositional disorder within the structure due to the mobility of Ag ions. [51][52][53]…”